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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 6 Mar 2024 16:09:32 -0800
Message-ID: <CAHk-=wgA1N72WfT9knweT=p1jhHGV3N0C2Z+7zvGL+LgG-AwXA@mail.gmail.com>
Subject: Re: [PATCH v2] x86: disable non-instrumented version of copy_mc when
 KMSAN is enabled
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Alexander Potapenko <glider@google.com>, Marco Elver <elver@google.com>, 
	Dmitry Vyukov <dvyukov@google.com>, kasan-dev <kasan-dev@googlegroups.com>, 
	LKML <linux-kernel@vger.kernel.org>, "the arch/x86 maintainers" <x86@kernel.org>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, "H. Peter Anvin" <hpa@zytor.com>
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On Wed, 6 Mar 2024 at 14:08, Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> Something like below one?

I'd rather leave the regular fallbacks (to memcpy and copy_to_user())
alone, and I'd just put the

        kmsan_memmove(dst, src, len - ret);

etc in the places that currently just call the MC copy functions.

The copy_mc_to_user() logic is already set up for that, since it has
to do the __uaccess_begin/end().

Changing copy_mc_to_kernel() to look visually the same would only
improve on this horror-show, I feel.

Obviously some kmsan person needs to validate your kmsan_memmove() thing, but

> Can we assume that 0 <= ret <= len is always true?

Yes. It had better be for other reasons.

                  Linus